Efavirenz and CYP2C9 Genetic Polymorphisms Reduce CYP2C9 Activity in Healthy Participants.

Area of Science:

• Pharmacology
• Drug Metabolism
• Clinical Pharmacology

Background:

• Cytochrome P450 2C9 (CYP2C9) is crucial for metabolizing various drugs.
• Efavirenz is an antiretroviral medication with potential drug-drug interaction (DDI) effects.
• The in vivo impact of efavirenz on CYP2C9 activity was previously uncharacterized.

Purpose of the Study:

• To investigate the effect of chronic efavirenz dosing on CYP2C9 activity in humans.
• To assess the clinical significance of efavirenz-CYP2C9 drug-drug interactions (DDIs).

Main Methods:

• A clinical DDI study using tolbutamide as a probe substrate for CYP2C9.
• Healthy participants received single (SD) and multiple doses (MD) of efavirenz.
• Pharmacokinetic analysis of tolbutamide and its metabolites, with CYP2C9 genotyping.

Main Results:

• Multiple-dose efavirenz significantly increased tolbutamide exposure (AUC) by over 50% (p < 0.0001).
• Efavirenz reduced tolbutamide clearance and metabolite formation.
• CYP2C9 genetic variants influenced tolbutamide elimination, though genotype-specific DDI effects were limited by sample size.

Conclusions:

• Chronic efavirenz administration inhibits CYP2C9 activity in vivo.
• This inhibition poses a risk for patients taking narrow-therapeutic-index CYP2C9 substrates.
• Therapeutic drug monitoring and dose adjustments are recommended for co-administered efavirenz and CYP2C9 substrates like warfarin.